SiO.sub.2 (silicon dioxide) or SiN (silicon nitride) is usually used as a dielectric material when producing a thin film capacitor for use in an integrated circuit (IC), such as a signal storing capacitor in a DRAM (Dynamic Random Access Memory). Recently, as demand for a more highly integrated semiconductor device is increasing, a dielectric material which has high dielectric constant and small leakage current is also increasingly sought after. This has led to today's active research on oxide dielectric thin films with high dielectric constant, made of such materials as STO (SrTiO.sub.3, strontium titanate), or (Ba, Sr) TiO.sub.3 (barium strontium titanate), either of which has a dielectric constant higher than conventional dielectric materials such as SiO.sub.2 and SiN.
For example, Japan Journal Applied Physics. Vol. 31 (1992, pp. 3025-3028, T. Kuroiwa et al.) discloses a study of electric properties of an oxide dielectric STO film which is made by employing an RF (radio frequency) sputtering method.
According to this publication, the oxide dielectric STO film is formed in the manner presented as follows. First, on an Si substrate, a thermally-oxidated Si film with a thickness of 300 nm and a Pt film with a thickness of 100 nm as a lower electrode are laminated in this order in a film forming chamber at a temperature of 600.degree. C. Thereafter, with the pressure of oxygen in the film forming chamber maintained at 26.6 Pa, an RF sputtering is conducted so that an oxide dielectric STO film with a thickness of 75 nm is formed on the Pt film. After the film formation, the substrate is cooled down in the oxygen atmosphere. To evaluate the electric properties of the oxide dielectric STO film, other Pt films as upper electrodes are formed on the oxide dielectric STO film at room temperature.
According to the evaluation, when a voltage of 2 V was applied to the STO film through the upper and lower electrodes, the oxide dielectric STO film had a leakage current of 4.0.times.10.sup.-9 A/cm.sup.2 and a dielectric constant of 205. It is reported as a result of the evaluation that the leakage current depended greatly on the oxygen defects contained in the film, while it did not depend on the dispersion of Pt atoms from the lower electrode to the oxide dielectric STO film. It is also reported that conditions of the oxygen atmosphere can be an important factor in determining the quality of an oxide dielectric STO film. To be more specific, the higher the pressure of the oxygen in the chamber during film formation, the less oxygen defects are formed in the oxide dielectric STO film.
An oxide dielectric thin film is generally manufactured by the following methods: a sputtering method, MOCVD (Metalorganic Chemical Vapor Deposition) method, a reactive vapor deposition method, and a laser ablation method. The following description briefly explains these methods. According to the sputtering method, ions are collided against a target in a low-pressure atmosphere so that atoms or molecules from the target are deposited on a substrate. According to the MOCVD method, a substrate is heated up so that a thin film is formed on the substrate based on a mutual thermal decomposition and a chemical reaction between an organometal compound and a group five hydrogen compound. According to the reactive vapor deposition method, when oxygen, for example, is utilized, a material is thermo-evaporated at the pressure of oxygen so that a reaction is caused between the vaporised material and the oxygen, and a product thus obtained through the reaction is deposited on the substrate. On the other hand, according to the laser ablation method which utilizes dense photons of a laser, a laser is projected on the surface of an evaporative material so that a chemical bond on the surface of the evaporative material is cut causing the evaporative material to be evaporated, thereby causing a thin film to be formed on the substrate.
After an oxide dielectric thin film is formed in an oxygen atmosphere and at a constant temperature by employing either of the above-mentioned methods, either of the following four conventional methods is usually applied as a post-treatment: a first conventional method of cooling the substrate while stopping the supply of oxygen; a second conventional method of cooling the substrate while leaving it in the same oxygen atmosphere that was used in the film formation process; a third conventional method of cooling the substrate in an oxygen atmosphere such as an oxygen gas or an oxygen plasma; and a fourth conventional method of taking the substrate out of the film forming chamber and applying a heat treatment to the substrate in an oxygen atmosphere.
However, if either of these conventional post-treatments is applied after forming an oxide dielectric thin film, the oxide dielectric thin film device thus obtained has such unfavorable properties as low dielectric constant, great leakage current, and small dielectric strength. It is considered that the reason is that oxygen is not sufficiently absorbed in the film during the film formation process, thereby causing a lot of oxygen defects in the crystal lattice of the oxide dielectric thin film. Regarding the fourth conventional post-treatment method, since the substrate is once taken out of the film forming chamber after the film formation, moisture and carbon dioxide in the air are adhered to the film surface, thereby obstructing the absorption of oxygen into the film. It is seen that this causes a lot of oxygen defects in the crystal lattice.
Thus, when an oxide dielectric thin film to which such a conventional post-treatment has been applied is used for producing an oxide dielectric thin film device, the following problem arises. It is impossible to steadily obtain an oxide dielectric thin film device having preferable dielectric properties, such as being superior in insulation, having dielectric constant as high as bulk's, and hardly exhibiting dielectric breakdown.
Then, it may be expected that such a problem can be solved by raising the oxygen pressure in the film forming process so that enough oxygen is absorbed. However, considering the principles of the film forming methods, the foregoing methods of film forming (the sputtering method, the MOCVD method, the reactive vapor deposition method, and the laser ablation method) is virtually infeasible. To be more specific, regarding the sputtering method, since the method, according to the principles, naturally tends to cause glow discharge by applying DC voltage or high frequency voltage, abnormal discharge tends to happen when, for example, gas of around 50 Pa or above is introduced. Therefore, it is difficult to obtain the stable glow discharge. As to the MOCVD method, the reactive vapor deposition method, and the laser ablation method, it is requisite that the vapor pressure of the materials is higher than the degree of vacuum. But, it is extremely difficult, when the pressure of oxygen is high, to achieve a vapor pressure of, for example, around 50 Pa or above by heating evaporative materials.